1. Field of the Invention
The present invention relates to an imaging apparatus which utilizes a piezoelectric-transformer high-voltage power supply, and a method for controlling the same.
2. Description of the Related Art
In an image-forming apparatus of an electrophotographic process type, reduction in the size and weight of a high-voltage power supply adapted for providing a direct-current bias voltage and the like involved in forming an image by a transfer section leads to reduction in the size and weight of the image-forming apparatus. Hence, in place of an electromagnetic transformer of a winding type, making use of a thin, light, and high-power piezoelectric transformer has been studied so as to serve as a transformer of a high-voltage power supply.
Use of a piezoelectric-transformer composed of a ceramic material enables the high-voltage power supply to generate a high voltage with a higher efficiency than that of the electromagnetic transformer, and in addition, to keep the distance between electrodes of the primary and secondary of the transformer separated, regardless of coupling between the primary and secondary. Accordingly, a mold coating especially for insulating the transformer can be eliminated, resulting in reduction in the size and weight of the high-voltage power supply. Such a piezoelectric-transformer high-voltage power supply is disclosed, e.g., in Japanese Patent Laid-Open No. 11-206113.
With respect to output vs. frequency, a piezoelectric-transformer generally has a characteristic having a bell-bottom shape exhibiting the maximum output voltage at its resonant frequency. Accordingly, changing the driving frequency allows the piezoelectric-transformer to variably control an output voltage. For example, changing the driving frequency from a value sufficiently high than the resonant frequency to a lower value (still higher than the resonant frequency) allows the piezoelectric-transformer to increase the output voltage.
In a comparator circuit, an output-voltage setting signal voltage and an output-voltage detecting signal are compared to each other. A voltage value of the compared result is input into a voltage control oscillator (VCO) and subjected to frequency conversion (V-F conversion), and the piezoelectric-transformer is driven at the frequency. The output voltage is raised or lowered by setting the output-voltage setting signal at a level corresponding to a desired target voltage or an OFF state of the output voltage, respectively. In order to achieve a normal operation of a feedback circuit for comparing the output-voltage setting signal and the output-voltage detecting signal to each other without causing a voltage control oscillator (VCO) to suffer from circuit oscillation in its control operation, a time constant of each of the output-voltage setting signal and the output-voltage detecting signal must be appropriately set.
Unfortunately, setting the time constant so as to prevent the circuit oscillation causes deterioration in a response of an output (a longer rising/falling time), results in an unacceptable response time (i.e., too long). For example, in cases where a high-voltage is supplied in a transfer roller of the image forming apparatus using the above-described piezoelectric-transformer high-voltage power supply, a faulty charging occurs in the transfer roller by the longer rising/falling time, thereby causing a faulty transfer. The faulty transfer causes image quality deterioration. Therefore, the longer rising/falling response causes technical problems when the image forming apparatus is applied to the piezoelectric-transformer high-voltage power supply.
FIG. 7 illustrates an output-voltage rising characteristic of a transfer section, wherein the resistance of a resistor and the capacitance of a capacitor, within the comparator are respectively set at 1 MΩ and 4700 pF so as to provide an integral circuit constant at the input stage of the output-voltage setting signal, and, in order to raise the output voltage from 0V to +3.5 kVDC, a step signal from 0V to about 10V, serving as the output-voltage setting signal Vcont is input. In FIG. 7, the horizontal and vertical axes respectively represent time and voltage, and the upper and lower curves respectively represent the output-voltage Vout and the output-voltage setting signal Vcont. In this case, the rising time of the output voltage from 0V to a target voltage +3.5 kV is about 100 milli-seconds (ms), which is longer than when a winding electromagnetic transformer is used (about 40 ms).
Therefore, the longer rising time inherent with the known control technique used with the piezoelectric-transformer high-voltage power supply in an image forming apparatus (as compared to electromagnetic transformers), produces an undesirable effect of deterioration in a response of an output (a longer rising/falling time).